The invention refers to micro swivel actuators and a procedure for the production of the same.
A multitude of micromechanical actuators, which aim at defined deflection of light beams by means of a mirror arrangement, is known in the literature.
An electrostatic deflector unit is described in the DE 42 24 599. The basis is a platelike swiveling element, which was reduced in mass either by realization in a sandwich structure or by recesses between remaining weblike areas. Suspension is provided by two flectional beams fitted in diagonally opposing corners. This element is held in a frame. The DD 298 856 two-dimensional micromechanical moving device contains another solution. The swivel plate is here centrically arranged on a point, which is located on a baseplate and is kept in the initial state parallel to the baseplate with four spring elements. The entire device is again arranged in a frame.
These solutions include individual tilting attachments, which are therefore limited in their use by the determined tilting surface. Furthermore, a relatively large area is being tilted, which results in another limitation in dynamics.
The EP 0 040 302 contains an optical beam deflector unit. A platelike deflector unit made of silicon is here electrostatically tilted with two one-sided torsion springs towards a baseplate. Moreover, this platelike deflector unit is preceded by parallel or non-parallel multiple arrangements, each of which is tightly enclosed by the frame.
The disadvantage here is again the large-surface realization of the tilting attachment. The result is limited dynamics. Solutions without a frame can be found in
EP 0 463 348 bistable DMD trigger circuit and trigger mode and PA1 EP 0 469 293 multilayer ductile mirror structure. Platelike mirrors are here tilted around the axis of symmetry.
The disadvantage of these solutions is a large mass and a high mass moment of inertia with regard to the geometrical axis. The preparation of a mirror array is described in the journal Sensors and Actuators, Vol. A 41 complete (1994), pages 324-329, "Lineaddressable torsional micromirrors for light modulator arrays." In order to achieve the base distance between the lower edges of the mirror and the electrodes, a sacrificial layer of silicon dioxide is applied after the production of the electrodes. Windows are etched in this sacrificial layer to mechanically fasten the springs, which enable the excursion of the mirrors on the substrate. The following separation of polysilicon therefore creates a layer which is in part directly connected with the substrate. The mirrors are then formed in those areas of the polysilicon layer that are lined with the sacrificial layer. Afterwards an aluminum layer is applied as reflector. Wet-chemical etching of the sacrificial layer is the last step in the manufacturing process. An overall coverage of mirror arrangement is not achieved.
The described surface technique utilizes a polysilicon layer for the actuators.
A development of Texas Instruments Inc. is presented in the journal Solid State Technology, July 1994, pages 63-68, "Digital micromirror array for projection TV." The array was especially designed for TV applications and consists of a matrix of 768.times.576 individual mirrors. The base distance between the mirrors and the electrodes is here implemented by an organic sacrificial layer, which is spun-on and then leveled to smooth out any unevenness created by underlying structures (electrodes). Mirrors as well as the springs are made of an aluminum alloy. Special posts are designed to support the springs towards the substrate. These posts are created by filling small holes in the sacrificial layer with the mentioned alloy. Following the decollation of the chips, the sacrificial layer is removed by plasma etching.
The described arrangements allow only two conditions for the mirrors (home position or else maximum excursion).
The invention mentioned in the patent claims 1 and 14 takes the problem of creating a micromechanical mirror array, featuring high attainable frequency of resonance and at the same time a large active total surface as a basis. This is to be realized by a procedure using fewer technological procedure steps.
This problem is solved with the characteristics described in the patent claims 1 and 14.
The special advantages achieved with this invention are that for one, a semiconductor wafer furnished with two layers of different etching characteristics contains all elements of preferably one mirror array, on the one hand by means of flexible, one-piece bandlike electrodes which are arranged parallel to each other, with integrated springs and supporting elements; and on the other hand by means of electrical arrangements of electrodes, feeder lines and bondpads, among others, without an existing conventional structure of assembled single layers.
The use of monocristalline silicon for the flexible electrodes enables the application of well-known and well-tried procedures of microelectronics. At the same time this results in high endurance of the springs.
The division of the entire tilting surface into flexible, one-piece bandlike electrodes, which are arranged parallel to each other, leads to an increase in the dynamics of the positioning processes due to the reduced masses and thus the lower angular momentum with regard to the swiveling axis. Furthermore, lower field intensities per flexible, one-piece bandlike electrodes, which are arranged parallel to each other, are necessary for moving.
The oxide layer directly on the semiconductor wafer constitutes a sacrificial layer. Therefore the forming of the support elements does not require any additional technological processes since these can be formed directly from the oxide layer by partial etching. This also makes the support elements self-adjusting with regard to the longitudinal axis of the mirror. The result is an even surface level of the support elements and therefore as well of the individual mirrors. Deformation and thus a falsification of the positioning result is prevented to a great extent. At the same time an interruption of the springs running inside the mirror is avoided, which also adds to the improvement of the properties. The second layer on the oxide layer is another insulation layer, which serves as carrier for the electrical arrangements so that with a turn of this semiconductor wafer the unevenness due to the forming of the electrodes of conventional micromechanical mirror arrangements becomes irrelevant.
The mechanical components are preferably made of monocristalline silicon, which is especially remarkable for its freedom from fatigue symptoms. Compared with polycristalline material, this enables almost unlimited durability of the mechanical components despite dynamic operation.
The individual flexible, one-piece bandlike electrodes, which are arranged parallel to each other, can be moved independently, so that with a formation of these electrodes as mirrors, the reflected beam is not only deflected, but can also be either focused or expanded. Furthermore they can be triggered analogously and therefore perform any moving function (among others sine, saw-tooth, triangle) around their longitudinal axis.
The use of narrow mirror surfaces as flexible, one-piece bandlike electrodes, which are arranged parallel to each other, enables the realization of bigger deviation angles than with the use of large-surface tilting mirrors with the same distance between the bottom edge of the mirror and the base frame. The narrower these electrodes can be formed, the bigger the possible deviation angles.
The arrangement of several micromechanical mirror arrays in one line or matrix enables a large surface deviation of beams combined with high dynamics.
The micromechanical mirror array is especially suited for applications in the pictorial reproduction technique via laser beam. A laser beam can here be directed with the turning motion of several parallel triggered individual mirrors. In order to achieve the high frequencies of resonance, the micromechanical mirror array with a large optically effective surface is composed of several parallel arranged individual mirrors of minor width and mostly of approximately the same length. The characteristic parameters of the arrangement, like frequency of resonance and deviation angle, are determined by the dynamic and static properties of the many small individual mirrors. The production of the micromechanical array utilizes procedure steps from the surface micromechanics, so that well-tried technologies are applied. This enables a production of these arrangements with existing facilities.
Favorable designs of the invention are mentioned in the patent claims 2 through 13 and 15 through 18.
The motions of the flexible, one-piece bandlike electrodes, which are arranged parallel to each other, have to be realized with the use of differently designed springs as swivel bearings according to further developments of the patent claims 2 through to 5. The use of a one-sided torsion spring according to the further development of patent claim 3 leads to a swivel bearing, allowing a motion of the electrode with the least possible expenditure of energy.
The further developments according to the patent claims 6 through 13 show functional realization alternatives. The use of reflection-reducing glass enables the use as deflection mirror unit.
The further development according to the patent claims 13, 16 and 17 allows the separation for the purpose of decollation of the micro swivel actuators from the wafer compound, since the semiconductor wafer is hermetically sealed by the glass wafer. This measure enables an effective mass production.
The further development according to patent claim 15 leads to an increase of the oxide layer as sacrificial layer, so that higher deflection angles can be obtained.